cmd/guru: emacs: only check modified buffers with file names
cmd/guru: emacs: only check modified buffers with file names

We cannot determine the extension of buffers with no file names (not
without throwing an error, anyway.)

Change-Id: I2b10390da738d71b6f95a6f668d3b92b54db29ed
Reviewed-on: https://go-review.googlesource.com/20470
Reviewed-by: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>

dominikh/go-mode.el,emacsmirror/go-mode